The invention relates to a synchronizable heart pacemaker having a timing circuit and an interference recognition circuit, which includes a testing stage for each signal, and a discriminator stage postcoupled to the testing stage, in which the presence or absence within a predetermined time interval of an output signal from the testing stage is determined. The predetermined time interval corresponds approximately to the QRS complex of a heart-action signal. As a result of this test the signal is either recognized as an interfering signal, and rendered ineffective, or is recognized as a heart-action signal, and fed to the timing circuit.
Pacemakers including interference recognition circuits of this kind operate according to the "principle of time analysis". In this principle of time analysis one starts from the fact that a heart-action signal has a characteristic form, which is characterized by a spike-like initial portion and a waveform following thereafter which is broader in shape than the spike-like initial portion, and wherein normally no further heart-action signal appears within a time interval from 180 to 200 ms following commencement of the heart-action signal. The principle of time analysis is therefore based on a test whether any further signal appears within the approximate duration of the Q-T-complex of the heart-action signal (refraction time or refractory period). If this is the case, then any further signal present is evaluated as an interfering signal.
In general current pacemakers, almost without exception, are in a position to take actions of the heart into account in a suitable manner. For example, one version of a pacemaker is programmed to discontinue operations, if the heart has a sufficiently high frequency, or rate of beats of its own (Demand pacemaker). Another version of a pacemaker attempts to implement or simulate the action of the auricle, and to generate therefrom a stimulation impulse for the ventricle, so as to ensure synchronization between the auricle and the chamber (auricle-controlled ventricular pacemaker). In spite of the physiological advantages systems of this type have to contend with the problem that any interfering signals could upset the circuit. Here it is necessary to differentiate between exogene and endogene interference sources. Exogene interference sources are due to magnetic, electrical, as well as electromagnetic fields, or are simply due to galvanic contacts of parts carrying any voltage. Endogene interferences are generated when any muscle signals, or the T-wave of the electrocardiogram, or, in the case of auricle electrodes, the R-wave of the ventricle are encountered in the auricle.
All interference-recognition circuits attempt to find differences between the enumerated sources of interference and the actual synchronizing heart signals, which are then required to be discriminated by electronic means. This is accomplished in practically all pacemakers now on the market by an active or a passive bandpass filter, which has a lower pass-frequency lying between 5 Hz and 20 Hz, and an upper pass-frequency lying between 50 Hz and 100 Hz. It has been shown, however, that a mere filtering of frequencies is not in a position to differentiate adequately between interfering signals, and operative signals, also primarily due to the fact that the most frequent interference is that of the mains supply (In Europe 50 Hz, and in the U.S. 60 Hz), which can almost certainly not be suppressed.
More effective discriminating methods can only be expected if the typical structure of an intracardiac signal, irrespective whether it is atrial or ventricular, is taken into account. A description of typical waveforms is given below, which applies when the heart signal is obtained in a unipolar manner (one electrode is disposed very close to the heart wall, the other electrode is very remote therefrom). In this case a very typical signal is registered, which starts with a more-or-less large positive peak, changes shortly thereafter to a negative peak, and wherein a positive peak of reduced steepness follows. A subsequent broader waveform, namely a waveform having a predominantly low-frequency content compared to that of the peaks, may have different shapes; for example, its positive portion can be relatively large, particularly in case of any first implantation, but this portion can also be missing altogether, particularly in the case of electrodes which have been implanted for some time. In each case there arises towards the end of the heart action again a somewhat broader negative wave, denoted as a T-wave. It has been found that, as an average, the fall time of the initial negative peak amounted to 4.6 ms (region between 0.5 and 10.9 ms), while the subsequent rise time amounted to, as an average 17.3 ms (region between 6.8 and 35 ms), so that the ratio between the fall time and the rise time is about 1:4. To this difference between the fall time and the rise time corresponds a difference between the steepness of flanks or trails of the negative-going and the positive-going signals, which difference has a ratio of 4:1. This typical structure can be explained theoretically in a manner such that a wave having the character of a dipole passes immediately along the electrode, whose positive charge initially results in a positive signal, but which, upon the positive charge moving immediately below the electrode, is quickly reshaped, so as to assume the negative extreme value when passing the negative charge. The further progress or shape of the wave is then determined by the far field of this dipole wave. Signals of the remote field do not have comparable steepnesses, so that examination relating to steepness of the flanks or trails permits the achievement of an effective discrimination between signals in the remote field (excitation of the respective other chamber, or muscle signals) and those of the near field.
In a known heart pacemaker circuit of this kind (U.S. Pat. No. 3,985,142), a test stage of the interference recognition circuit consists of a comparator circuit in which the input signal is compared to a reference signal, and wherein the output signal of the comparator circuit is dependent on the result of this comparison.
In this circuit each incoming signal, regardless whether it is an interfering signal, or an operative signal, starts an expecting interval. Following a short refraction time, a test is performed during a relatively short interval, whether a further signal is present or not. In the event such a signal is present, it is evaluated as an interference signal, and an impulse is transmitted at the end of the expecting interval, independent of the fact what type of signal has been obtained from the heart.
It is an advantage of this circuit that each continuous, but also each pulse interference which lasts beyond the refraction time, is recognized by the circuit as an interference, and does not permit the pacemaker to stop operation in such cases.
It is a disadvantage, however, that each commencement of an interference is evaluated as a heart action, and that the circuit is overpowered, if the interference lasts for a time shorter than the built-in refraction time. Any non-recurring signals cannot be recognized as interfering signals.
In summary then, this known interference recognition circuit recognizes continuous and pulsed interfering signals, if these signals continue beyond the predetermined time interval. But it is not in a position to recognize any symmetrical interfering signals as such, if these correspond in their time duration to the heart-action signals, or are shorter than the refraction time.
Another known interference recognition circuit operates according to the "principle of the maximum searcher" (U.S. Pat. No. 3,927,677). In this known circuit for guarding against any interference, a voltage is generated according to the rise of the signal at the input of the amplifier, which includes a peak value storage stage for this purpose. The maximum value is maintained in a capacitor, which is discharged at a certain time constant. Each incoming interfering signal causes such a charging process, and each further interfering signal results in the stored value of the maximum searcher not being changed.
Only when a change of amplitude occurs, such as is the case, for example, when the heart signal is superimposed on an interfering signal, is a signal generated at the output of the peak value storage stage, which only triggers a subsequent timing circuit, when the amplitude exceeds a certain value.
It is an advantage of this circuit that heart signals can be recognized as such, even in the presence of any massive continuous interference.
But it is a disadvantage of a heart pacemaker which includes this interference recognition circuit, that it is not in a position to recognize all amplitude-modulated interference signals as such. An amplitude-modulated interfering signal, which generates at the output of the maximum searcher an adequately large signal, is rather evaluated in this circuit as a heart action signal, and consequently silences the pacemaker.
Experiments conducted by applicant have shown that in the case of an interference which has an amplitude of 100 mV, a rectangular amplitude-modulation of 1.5 mV is in a position to inhibit the pacemaker. As continuous interferences, in practice, are, in fact, a rare exception, this interference recognition circuit basically endangers patients, which necessarily have to rely on this pacemaker. For this reason the manufacturer of this pacemaker has drawn the appropriate conclusions and has no longer utilized the pacemaker in the aforedescribed manner since the end of 1979.
Reference should also be had to applicant's publication "Stoerbeeinflussung von Herzschrittmachern" (Impairment of heart-pacemakers by Interference), Herzschrittmacher 2, 1982, EBM GmbH.